Power storage system

ABSTRACT

A power storage system includes an AC/DC converter, a first control device, a power storage device, and a load. The first control device includes a measuring portion that measures the amount of power consumed by the load, a predicting portion that predicts the demand for power consumed by the load on the basis of the amount of power consumed by the load, and a planning portion that makes a charge and discharge plan of the power storage device on the basis of the demand for power predicted by the predicting portion. The power storage device includes a second control device, a DC/DC converter, a first battery cell group, and a second battery cell group. The power storage device is placed in an underfloor space surrounded by a base and a floor of a building.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a secondary battery and a power storagesystem using a secondary battery. The present invention relates to asystem having a function of recovering a secondary battery from itsdeteriorated state.

2. Description of the Related Art

In recent years, a variety of power storage devices such as lithium-ionsecondary batteries, lithium-ion capacitors, and air batteries have beenactively developed. In particular, demand for lithium-ion secondarybatteries with high output and high energy density has rapidly grownwith the development of the semiconductor industry, for electricaldevices, for example, portable information terminals such as mobilephones, smartphones, and laptop computers, portable music players, anddigital cameras. The lithium-ion secondary batteries are essential asrechargeable energy supply sources for today's information society.

If power supply equipment malfunctions or is partly broken or anelectric power company stops or reduces power supply because of naturaldisasters (e.g., crustal alteration such as earthquakes and groundsubsidence, typhoons, and lighting strikes), terrorism, accidents, orthe like, for example, not only social life but also personal lives canbe significantly influenced. Thus, demand of home-use power storagedevices that can provide electric energy by individuals has beenincreasing.

A lithium-ion secondary battery includes at least a positive electrode,a negative electrode, and an electrolytic solution (Patent Document 1).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2012-009418

SUMMARY OF THE INVENTION

It is preferable that home-use power storage devices have high capacityand a long lifetime. However, increasing the capacity of the powerstorage devices increases the volume thereof. Further, increasing thelifetime of the power storage devices decreases the volumetricefficiency, leading to an increase in the volume. When the volume of thepower storage devices is increased, it is difficult to provide the powerstorage device indoors; thus, the power storage device needs to beprovided outdoors.

However, in the case where the power storage device is providedoutdoors, it is exposed to rain or the like and thus deteriorates bymoisture. Further, in the case where the power storage device isprovided outdoors, when the outside air is at a low temperature (e.g.,in a minus temperature range), the power storage device significantlydeteriorates, reducing the lifetime of the power storage device.Inhibiting deterioration of the power storage device requires regularmaintenance of the power storage device; thus, in addition to the costfor purchasing the power storage device, the maintenance cost and thelike are further needed. Consequently, burdens of the cost of a powerstorage device are large for an individual.

In the case where an individual possesses a home-use power storagedevice and installs it outside his/her residence (building), the powerstorage device is provided in an area of a site that can be used by theindividual. When the ratio of the building area to the site area, thatis, building-to-land ratio, is high and the power storage device isprovided in a space limited by an adjacent house or a wall, the size ofthe power storage device is necessarily considered. Even in the casewhere there is an enough outside space for installation of a large powerstorage device, it may be difficult to secure a carrying path forinstallation. Note that the term “outside a house” mean an area otherthan the building area when seen from above.

In view of the above problems, an object of one embodiment of thepresent invention is to provide a power storage device with highcapacity. Another object of one embodiment of the present invention isto provide a power storage device with a long lifetime. Another objectof one embodiment of the present invention is to provide a power storagedevice with high reliability.

Further, another object of one embodiment of the present invention is toprovide a small-sized power storage device with high capacity that canbe provided indoors.

One embodiment of the present invention is a power storage systemincluding an AC/DC converter, a first control device, a power storagedevice, and a load. The first control device includes a measuringportion that measures the amount of power consumed by the load, apredicting portion that predicts the demand for power consumed by theload on the basis of the amount of power consumed by the load, and aplanning portion that makes a charge and discharge plan of the powerstorage device on the basis of the demand for power predicted by thepredicting portion. The power storage device includes a second controldevice, a DC/DC converter, a first battery cell group, and a secondbattery cell group. One terminal of the DC/DC converter is electricallyconnected to one terminal of the AC/DC converter and one terminal of thefirst battery cell group. The other terminal of the DC/DC converter iselectrically connected to the one terminal of the AC/DC converter andone terminal of the second battery cell group. The power storage deviceis placed in an underfloor space surrounded by a base and a floor of abuilding.

According to one embodiment of the present invention, a power storagedevice with high capacity can be provided. A small-sized power storagedevice with high capacity that can be provided indoors can be provided.A power storage device with a long lifetime can be provided. Thereliability of a power storage device can be improved. A power storagesystem using the power storage device can be provided.

When power storage devices of one embodiment of the present inventionare widely used, as the number of houses that install the power storagedevices of one embodiment of the present invention indoors increases,burdens of a power plant in a region where the houses are located arereduced, which can contribute to an effective use and a stable supply ofpower. Further, according to one embodiment of the present invention,the power storage device is charged in the night time when the useamount of power is small, and is used in the day time when the useamount of power is large; thus, power can be efficiently stored andused. Further, since the power storage device 101 is used in the daytime when the usage charge of a commercial power source is high, theelectricity charge is low and an economic merit can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B each illustrate a power storage device placed in anunderfloor space of a building;

FIGS. 2A and 2B are conceptual diagrams showing the state where alithium-ion secondary battery is charged;

FIGS. 3A and 3B are conceptual diagrams showing the state where alithium-ion secondary battery is discharged;

FIG. 4 illustrates the relation between the potentials of a positiveelectrode and a negative electrode;

FIGS. 5A to 5F are schematic cross-sectional views illustrating oneembodiment of the present invention;

FIGS. 6A to 6F are schematic cross-sectional views illustrating oneembodiment of the present invention;

FIGS. 7A to 7F are schematic cross-sectional views illustrating oneembodiment of the present invention;

FIG. 8 is a schematic diagram illustrating a power storage system of oneembodiment of the present invention;

FIG. 9A shows the amount of power consumed by a load, and FIGS. 9B and9C show electric rates depending on time periods;

FIG. 10 is a circuit diagram of a power storage device;

FIGS. 11A and 11B illustrate a positive electrode;

FIGS. 12A and 12B illustrate a negative electrode;

FIGS. 13A to 13C each illustrate a battery cell;

FIG. 14A is a graph showing the thicknesses of components of batterycells, and FIG. 14B is a graph showing the cell capacities of thebattery cells;

FIGS. 15A and 15B are graphs each showing cycle characteristics; and

FIGS. 16A and 16B are graphs each showing cycle characteristics.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments and an example of the present invention will be describedbelow in detail with reference to the drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways. The present invention is notconstrued as being limited to descriptions of the embodiments and theexample.

Embodiment 1

In this embodiment, a power storage device of one embodiment of thepresent invention will be described with reference to FIGS. 1A and 1B.

A building 100 illustrated in FIG. 1A includes a base 102, a floor 103,an exterior wall 104, a space 105, and an underfloor space 106. A powerstorage device 101 (also referred to as an electronic device) of oneembodiment of the present invention is placed in the underfloor space106, which is surrounded by the base 102 and the floor 103 of thebuilding 100.

As illustrated in FIG. 1B, the underfloor space 106 is partitioned bythe bases 102 in the building 100. The inside of the building 100 ispartitioned by an interior wall 107. The power storage device 101 isplaced in the underfloor space 106. In the case where there are aplurality of underfloor spaces 106 partitioned by the bases 102, thepower storage device 101 can be placed in each of the underfloor spaces106.

The power storage device 101 of one embodiment of the present inventionincludes a plurality of secondary batteries (hereinafter also referredto as battery cells).

A battery cell of one embodiment of the present invention preventsdeposition and growth of a reaction product on a surface of anelectrode, which might cause deterioration or a variety of abnormalsituations. Even if a reaction product is deposited, the reactionproduct can be dissolved by applying a signal to supply a current in thereverse direction of a current with which a reaction product is formed.Further, the growth of a reaction product can be inhibited by applying asignal to supply a current in the reverse direction of the current withwhich a reaction product is formed before formation of a reactionproduct. This can inhibit deterioration of the electrode of a batterycell.

The signal with which a reverse current flows refers to a pulse current,and can also be referred to as an inversion pulse current. Note that aninversion pulse current refers to a current of a signal with which acurrent does not flow successively or continuously but flows momentarilyor continuously only for a moment (for 0.1 seconds to 3 minutesinclusive, typically 3 seconds to 30 seconds inclusive). Intervals atwhich an inversion pulse current is supplied and the intensity of theinversion pulse current are set as appropriate.

The use of a plurality of such battery cells in the power storage device101 can increase the reliability of the power storage device 101. Thiscan increase the lifetime of the power storage device 101, so thatregular maintenance of the power storage device 101 is not required orthe frequency of maintenance can be reduced. When regular maintenance ofthe power storage device 101 of one embodiment of the present inventionis not required, a workspace for maintenance around the power storagedevice 101 is unnecessary. Accordingly, the power storage device 101 canbe provided indoors. Note that in this specification and the like, theterm “provided indoors” means being provided in the area of a building(except a rooftop) when seen from above, and being provided in anunderfloor space or a basement of the building is covered by the term.

Further, repeatedly supplying an inversion pulse current while thebattery cell is charged can prevent heat generation or ignition of thebattery cell due to a short circuit caused by a reaction productdeposited on a surface of an electrode of the battery cell. In otherwords, since the safety of the power storage device 101 including thebattery cell can be improved, the power storage device 101 can be placed(or provided) in the underfloor space 106.

In order to ensure the safety of the power storage device 101 morereliably, an exterior of the power storage device 101 preferably hasmeasures against water and fire. Further, the base 102 and the floor 103preferably have measures against water and fire.

For example, lithium metal deposited on a negative electrode of abattery cell causes a variety of defects in the battery cell, whichmight decrease the reliability of the battery cell. In order to preventthis, the capacity of a negative electrode is made high with respect tothe capacity of a positive electrode (the capacity ratio is set to below) in some cases. The capacity ratio means the proportion of thevolume capacity of a positive electrode to the volume capacity of anegative electrode, and when the capacity ratio can be high, the wholevolume capacity (the total of the volume capacity of the positiveelectrode and the volume capacity of the negative electrode) withrespect to a certain capacity value can be low.

Even if lithium metal is deposited on a negative electrode, it can bedissolved by supplying an inversion pulse current while a battery cellis charged; thus, the reliability of the battery cell can be increased.Consequently, the capacity ratio of the battery cell can be increased,so that the size of the battery cell can be reduced.

Thus, the significantly downsized power storage device 101 that usessuch downsized battery cells can be installed indoors; specifically, thepower storage device 101 can be placed in the underfloor space 106.Further, the power storage device 101 using such downsized battery cellscan have a longer lifetime; thus, replacement of the power storagedevice 101 is unnecessary. Further, the size of the power storage device101 may be increased to a size such that it can be placed in theunderfloor space 106.

The use of a plurality of battery cells of one embodiment of the presentinvention precludes an increase in the volume of the power storagedevice 101 as large as an increase in that of a conventional one; thus,the power storage device 101 does not need to be provided outdoors andcan be placed in the underfloor space 106. When being placed in theunderfloor space 106 as described above, the power storage device 101can be prevented from being exposed to rain or the like, inhibitingdeterioration of the power storage device 101 due to moisture. Further,even when the outside air is at a low temperature (e.g., in a minustemperature range), deterioration of the power storage device 101 can beinhibited because the power storage device 101 is provided indoors. Thiscan further increase the lifetime of the power storage device 101.

The lifetime of a building such as an individual house is approximately30 years after construction. The power storage device 101 of oneembodiment of the present invention is placed in the underfloor space106 and is preferably kept placed in the underfloor space 106 for 30years, more preferably, for 50 years, without maintenance.

The power storage device 101 can be provided in the underfloor space 106when or after the building 100 is built. The underfloor space 106 of thebuilding 100 can be effectively used.

In the case where a conventional power storage device is providedoutdoors, a wide area for providing the power storage device is needed.Further, when the power storage device is provided, it is necessary tosecure a workspace for maintenance around the power storage device.

When the power storage device 101 of one embodiment of the presentinvention is provided in the underfloor space 106, a wide area forproviding the power storage device 101 outdoors is unnecessary. Further,when the power storage device 101 is provided in the underfloor space106, it is not necessary to secure a workspace for maintenance aroundthe power storage device 101.

The amount of power that can be stored in the power storage device 101of one embodiment of the present invention is greater than or equal to10 kWh and less than or equal to 40 kWh. In the case where the amount ofpower that can be stored in the power storage device 101 is 40 kWh, forexample, 400 battery cells with 100 Wh at 3.2 V are used. Since the sizeof the battery cell can be reduced as described above, an increase inthe volume of the power storage device 101 can be prevented even whenthe number of battery cells needed for the power storage device 101 isincreased to more than 400. It is preferable that, for example, agraphite electrode and lithium iron phosphate (LiFePO₄) be used for anegative electrode and a positive electrode of the battery cell,respectively. In that case, the safety of the battery cell and thesafety of the power storage device 101 including the battery cell can beincreased.

For example, the power storage device of one embodiment of the presentinvention is charged using an AC/DC converter in the night time anddischarged using a DC/AC converter (e.g., 50 Hz or 60 Hz) in the daytime. The power storage device 101 is charged in the night time when theuse amount of power is small, and is used indoors in the day time whenthe use amount of power is large; thus, power can be efficiently storedand used. Further, since the power storage device 101 is used in the daytime when the usage charge of a commercial power source is high, theelectricity charge is low and an economic merit can be obtained. Notethat the frequency and voltage at the time of using power stored in thepower storage device 101 can be set as appropriate depending on a region(country) where the power storage device 101 is used.

As illustrated in FIG. 1B, the power storage device 101 is provided witha control device 110, and the control device 110 is electricallyconnected to a distribution board 109 through a wiring 111.

The control device 110 has a function of controlling charge anddischarge of each battery cell, a function of protecting the batterycells from overcurrent and overvoltage, a function of controllingtemperature, a function of controlling a battery balance between thebattery cells, a function of outputting power to the distribution board109, and the like.

A plurality of power storage devices 101 provided in the underfloorspaces 106 under rooms each may include the control device 110, and eachof the control devices 110 may be electrically connected to thedistribution board 109 through the wiring 111.

In the power storage device 101 illustrated in FIGS. 1A and 1B, aplurality of battery cells which operate according to the mechanismsdescribed above are connected in series. Further, parallely connectingunits each including the plurality of battery cells (one unit is alsoreferred to as a battery cell group) connected in series can increasethe capacity of the power storage device 101. Even when having highcapacity and large volume, the power storage device 101 can be placed inan underfloor space surrounded by a base and a floor of a building asillustrated in FIGS. 1A and 1B. Since the power storage device 101 canbe placed in the underfloor space 106, the power storage device 101 doesnot need to be provided outdoors. When being placed in the underfloorspace 106 as described above, the power storage device 101 can beprevented from being exposed to rain or the like, inhibitingdeterioration of the power storage device 101 due to moisture. Further,even when the outside air is at a low temperature (e.g., in a minustemperature range), deterioration of the power storage device 101 can beinhibited because the power storage device 101 is provided indoors. Thiscan further increase the lifetime of the power storage device 101.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 2

In this embodiment, the operation principle of a lithium-ion secondarybattery will be described and then a mechanism of product formation onan electrode surface of a battery cell and a mechanism of productdissolution will be described.

First, the operation principle of the lithium-ion secondary battery willbe described with reference to FIGS. 2A and 2B, FIGS. 3A and 3B, andFIG. 4.

FIGS. 2A and 2B show the case of charging the lithium-ion secondarybattery, and FIGS. 3A and 3B show the case of discharging thelithium-ion secondary battery. As illustrated in FIG. 2A and FIG. 3A,when the battery is regarded as a closed circuit, lithium ions transferand a current flows in the same direction. Further, in the lithium-ionsecondary battery, an anode and a cathode change places in charge anddischarge, and an oxidation reaction and a reduction reaction occur onthe corresponding sides; hence, an electrode with a high redox potentialis called a positive electrode and an electrode with a low redoxpotential is called a negative electrode in this specification. For thisreason, in this specification, the positive electrode is referred to asa “positive electrode” and the negative electrode is referred to as a“negative electrode” in all the cases where charge is performed,discharge is performed, an inversion pulse current is supplied, adischarging current is supplied, and a charging current is supplied. Theuse of the terms “anode” and “cathode” related to an oxidation reactionand a reduction reaction might cause confusion because the anode and thecathode change places at the time of charging and discharging. Thus, theterms “anode” and “cathode” are not used in this specification. If theterm “anode” or “cathode” is used, it should be mentioned that the anodeor the cathode is which of the one at the time of charging or the one atthe time of discharging and corresponds to which of a positive electrodeor a negative electrode. In FIGS. 2A and 2B and FIGS. 3A and 3B, apositive electrode includes lithium iron phosphate (LiFePO₄) as apositive electrode active material, and a negative electrode includesgraphite as a negative electrode active material.

FIG. 2A illustrates a lithium-ion secondary battery 201 and a charger202 when the lithium-ion secondary battery is charged. In charging thelithium-ion secondary battery, a reaction expressed by Formula (1)occurs in the positive electrode.

LiFePO₄→FePO₄+Li⁺+e⁻  (1)

In addition, a reaction expressed by Formula (2) occurs in the negativeelectrode.

C₆+Li⁺+e⁻→LiC₆   (2)

Thus, the overall reaction in charging the lithium-ion secondary batteryis expressed by Formula (3).

LiFePO₄+C₆→FePO₄+LiC₆   (3)

The battery is supposed to be charged when lithium ions are intercalatedinto graphite in the negative electrode; however, in the case where alithium metal is deposited on the negative electrode because of anycause, a reaction expressed by Formula (4) occurs. That is, both areaction of lithium intercalation into graphite and a lithium depositionreaction occur at the negative electrode.

Li⁺+e⁻→Li   (4)

The equilibrium potentials of the positive electrode and the negativeelectrode are determined by a material and an equilibrium state of thematerial. The potential difference (voltage) between the electrodesvaries depending on the equilibrium states of the materials of thepositive electrode and the negative electrode.

FIG. 2B shows voltage when the lithium-ion secondary battery is charged.As shown in FIG. 2B, in charging, as a reaction proceeds due to acurrent that flows over time t, the voltage between the electrodesincreases.

FIG. 3A illustrates the lithium-ion secondary battery 201 and a load 203when the lithium-ion secondary battery is discharged. When thelithium-ion secondary battery is discharged, a reaction expressed byFormula (5) occurs in the positive electrode.

FePO₄+Li⁺+e⁻→LiFePO₄   (5)

In addition, a reaction expressed by Formula (6) occurs in the negativeelectrode.

LiC₆→C₆+Li⁺+e⁻  (6)

Thus, the overall reaction in discharging the lithium-ion secondarybattery is expressed by Formula (7).

FePO₄+LiC₆→LiFePO₄+C₆   (7)

In addition, in discharge performed after the lithium metal isdeposited, a reaction expressed by Formula (8) occurs in the negativeelectrode. That is, both a reaction of lithium deintercalation fromgraphite and a lithium dissolution reaction occur at the negativeelectrode.

Li→Li⁺+e⁻  (8)

FIG. 3B shows voltage when the lithium-ion secondary battery isdischarged. As shown in FIG. 3B, in discharging, as a reaction proceedsdue to a current that flows over time t, the voltage between theelectrodes decreases.

FIG. 4 illustrates the relation between the electrode potential of apositive electrode including lithium iron phosphate and the electrodepotential of an electrode including a lithium metal, and the relationbetween the electrode potential of a negative electrode includinggraphite and the electrode potential of the electrode including alithium metal. In FIG. 4, the hollow arrow represents a chargingvoltage.

The potential difference between the positive electrode includinglithium iron phosphate and the negative electrode including graphite isas follows: 3.5 V-0.2 V=3.3 V. At a charging voltage of 3.3 V, thereaction of Formula (1) and the reaction of Formula (5) equilibrate inthe positive electrode and the reaction of Formula (2) and the reactionof Formula (6) equilibrate in the negative electrode; thus, a currentdoes not flow.

For this reason, a charging voltage higher than 3.3 V needs to beapplied so that a charging current is supplied. For example, on theassumption that a series resistance component inside the battery isignored and all extra charging voltage is used in the electrodereactions of Formulae (1) and (2), as indicated by the hollow arrow inFIG. 4, the extra charging voltage is shared by the positive electrodeand the negative electrode as an overvoltage to the positive electrodeand an overvoltage to the negative electrode. To obtain a higher currentdensity per unit electrode area, a higher overvoltage is necessary. Forexample, when the battery is rapidly charged, a current density per unitsurface area of an active material needs to be high; thus, a higherovervoltage is needed.

However, as the overvoltage is raised to increase the current densityper unit surface area of the active material, the overvoltage to thenegative electrode increases; therefore, a potential shown by the tip ofthe hollow arrow in FIG. 4 becomes lower than the potential of thelithium metal electrode. Then, the reaction of Formula (4) occurs. Atthis time, lithium is deposited on the surface of the negativeelectrode. Lithium deposited on the surface of the negative electrode isa product.

Next, a mechanism of reaction product formation on an electrode surfaceof a battery cell and a mechanism of reaction product dissolution willbe described with reference to FIGS. 5A to 5F, FIGS. 6A to 6F, and FIGS.7A to 7F.

FIGS. 5A to 5C are schematic cross-sectional views sequentiallyillustrating the states of an electrode 210 of a battery, specifically,the states of reaction products 212 a, 212 b, and 212 c abnormally grownon a surface of a negative electrode.

FIG. 5A is the schematic view of part of the battery including at leasta positive electrode, a negative electrode, and an electrolyticsolution.

Only the one electrode 210 and an electrolytic solution 213 in thevicinity of the electrode 210 are illustrated in FIGS. 5A to 5C forsimplicity.

In FIGS. 5A to 5F, the electrode 210 is either a positive electrode or anegative electrode, and descriptions will be given on the assumptionthat the electrode 210 is a negative electrode. FIG. 5A illustrates thestate where a current is supplied between the negative electrode and apositive electrode (not illustrated) during a period t1 and the reactionproducts 212 a are deposited on the electrode 210 (negative electrode)so that the electrode 210 is dotted with the reaction products 212 a.

FIG. 5B illustrates the state where a current is supplied between thenegative electrode and the positive electrode during a period t2 (t2 islonger than t1). Projections of the reaction product 212 b areabnormally grown from the positions where they are deposited and thereaction product 212 b is deposited on the entire surface of theelectrode.

FIG. 5C illustrates the state where a current is supplied during aperiod t3 longer than the period t2. Projections of the reaction product212 c in FIG. 5C are grown to be longer than the projections of thereaction product 212 b in FIG. 5B in the direction perpendicular to theelectrode surface. Although an example of a reaction product grown tolengthen in the direction perpendicular to the electrode surface isillustrated in FIG. 5B, one embodiment of the present invention is notparticularly limited to the example; the reaction product may have abent portion or a plurality of bent portions while being grown. Athickness d2 of the projection of the reaction product 212 c is largerthan or equal to a thickness d1 of the projection of the reactionproduct 212 b illustrated in FIG. 5B.

A reaction product is not uniformly deposited on the entire surface ofthe electrode as a current supply time passes. A reaction product ismore likely to be deposited on the position where a reaction product hasbeen deposited than on the other positions, and a larger amount ofreaction product is deposited on the position and grown to be a largelump. The region where a large amount of reaction product has beendeposited has a higher conductivity than the other region. For thisreason, a current is likely to concentrate at the region where the largeamount of reaction product has been deposited, and the reaction productis grown around the region faster than in the other region. Accordingly,a projection and a depression are formed by the region where a largeamount of reaction product is deposited and the region where a smallamount of reaction product is deposited, and the projection and thedepression become larger as time goes by as illustrated in FIG. 5C.Finally, the large projection and depression cause severe deteriorationof the battery.

After the state in FIG. 5C, a signal to supply a current in the reversedirection of the current with which a reaction product is formed, aninversion pulse current here, is applied to dissolve the reactionproduct. FIG. 5D illustrates the state at the time immediately after theinversion pulse current is supplied. As shown by arrows in FIG. 5D, areaction product 212 d is dissolved from its tip or surface. This isbecause when the inversion pulse current is supplied, the potentialgradient around the tip or surface of the reaction product becomessteep, so that the tip or surface of the reaction product is likely tobe preferentially dissolved.

The inversion pulse current with which a current flows in the reversedirection of the current with which a reaction product is formed issupplied in the state where the projection and depression due tonon-uniform deposition of a reaction product are formed, whereby acurrent concentrates at the projection and the reaction product isdissolved. The reaction product dissolution means that a reactionproduct in a region in the electrode surface where a large amount ofreaction product is deposited is dissolved to reduce the area of theregion where a large amount of reaction product is deposited, preferablymeans that the electrode surface is restored to the state at the timebefore a reaction product is deposited on the electrode surface. As wellas restoring the electrode surface to the state at the time before areaction product is deposited on the electrode surface, even inhibitingthe growth of a reaction product so that it is kept small or reducingthe size of a reaction product can produce a significant effect.

FIG. 5E illustrates a state where the reaction product is beingdissolved owing to further flow of the inversion pulse current; thereaction product 212 d is dissolved from its tip or surface to be thereaction product 212 e smaller than the reaction product 212 d.

Then, a signal to supply a current in the reverse direction of thecurrent with which a reaction product is formed, e.g., an inversionpulse current, is applied one or more times, so that ideally, theelectrode surface can be restored to the state at the time before areaction product is deposited on the electrode surface as illustrated inFIG. 5F. Since a charging current flows from the right side to the leftside in FIGS. 5A to 5F, an inversion pulse current is supplied so as toflow in the direction opposite to the direction of the current flow(from the left side to the right side in FIGS. 5A to 5F). Specifically,one period during which the inversion pulse current is supplied islonger than or equal to 0.1 seconds and shorter than or equal to 3minutes, typically longer than or equal to 3 seconds and shorter than orequal to 30 seconds.

A technical idea of one embodiment of the present invention is toutilize the mechanism of reaction product formation and the mechanism ofreaction product dissolution. One embodiment of the invention disclosedin this specification and the like includes a first electrode and asecond electrode, and includes at least an electrolytic solution betweenthe first electrode and the second electrode. A reaction product grownfrom at least one point in a surface of the first electrode when acurrent is supplied between the first electrode and the second electrodeis dissolved from a tip or a surface of the reaction product bysupplying a current in the reverse direction of the current with which areaction product is formed. The use of the mechanisms can provide anovel battery cell based on an extremely novel principle and a powerstorage system using the battery cell.

Another embodiment of the present invention is to apply a signal tosupply a current in the reverse direction of a current with which areaction product is formed, more than once. This embodiment of thepresent invention includes a first electrode and a second electrode, andincludes at least an electrolytic solution between the first electrodeand the second electrode. A reaction product grown from at least onepoint in a surface of the first electrode when a current is suppliedbetween the first electrode and the second electrode is dissolved from atip or a surface of the reaction product by supplying a current in thereverse direction of the current with which a reaction product isformed. Then, supply of a current between the first electrode and thesecond electrode and supply of a current in the reverse direction of thecurrent with which a reaction product is formed are repeated.

Another embodiment of the present invention is to apply a signal tosupply a current in the reverse direction of a current with which areaction product is formed, for a period shorter than a period duringwhich the reaction product is formed. This embodiment of the presentinvention includes a first electrode and a second electrode, andincludes at least an electrolytic solution between the first electrodeand the second electrode. A reaction product grown from at least onepoint in a surface of the first electrode when a current is suppliedbetween the first electrode and the second electrode for a predeterminedperiod is dissolved from a tip or a surface of the reaction product bysupplying a current in the reverse direction of the current with which areaction product is formed for a period shorter than the predeterminedperiod.

In the case where the reaction product is dissolved into theelectrolytic solution at high speed, the state in FIG. 5D can be changedinto the state in FIG. 5F even with an extremely short time ofapplication of a signal to supply a current in the reverse direction ofthe current with which a reaction product is formed.

Depending on the application condition (e.g., pulse width, timing, orintensity) of a signal to supply a current in the reverse direction ofthe current with which a reaction product is formed, even with onlyone-time supply of such a signal, the state in FIG. 5D can be changedinto the state in FIG. 5F.

Although the negative electrode is taken as an example in FIGS. 5A to5F, the above description can also apply to the positive electrode toachieve a similar effect.

The progress of the deterioration of a battery can be prevented or thebattery can be recovered from its deteriorated state by supplying asignal to supply a current in the reverse direction of a chargingcurrent with which a reaction product is formed.

One embodiment of the present invention is not limited to the mechanismillustrated in FIGS. 5A to 5F, and some other mechanisms are alsoembodiments of the present invention. Hereinafter, variations ofmechanisms will be described.

FIGS. 6A to 6F illustrate a mechanism partly different from that inFIGS. 5A to 5F in the process of generation (or the process of growth)of a reaction product; the reaction product is deposited on an entiresurface of an electrode and is partly grown abnormally.

FIGS. 6A to 6C are schematic cross-sectional views sequentiallyillustrating the states of an electrode 220, specifically, the states ofreaction products 222 a, 222 b, and 222 c abnormally grown on a surfaceof a negative electrode. A space between a pair of electrodes is filledwith an electrolytic solution 223.

FIG. 6A illustrates the state where a current is supplied between thenegative electrode and a positive electrode (not illustrated) during theperiod t1, and the reaction products 222 a are deposited on the entiresurface of the electrode 220 serving as the negative electrode andpartly grown abnormally. Examples of a material of the electrode 220 onwhich the reaction product 222 a is deposited are graphite, acombination of graphite and graphene oxide, and titanium oxide.

FIG. 6B illustrates the state of the reaction product 222 b grown when acurrent is supplied between the negative electrode and the positiveelectrode during the period t2 (t2 is longer than t1). FIG. 6Cillustrates the state of the reaction product 222 c grown when a currentis supplied during the period t3 longer than the period t2.

After the state in FIG. 6C, a signal to supply a current in the reversedirection of the current with which a reaction product is formed isapplied to dissolve the reaction product. FIG. 6D illustrates the stateat the time immediately after a signal to supply a current in thereverse direction of the current with which a reaction product isformed, e.g., an inversion pulse current, is applied. As shown by arrowsin FIG. 6D, a reaction product 222 d is dissolved from its tip orsurface.

FIG. 6E illustrates a state where the reaction product is beingdissolved owing to further flow of the inversion pulse current; thereaction product 222 d is dissolved from its tip or surface to be thereaction product 222 e smaller than the reaction product 222 d.

In this manner, one embodiment of the present invention can be appliedregardless of the process of generation of the reaction product and themechanism thereof. A signal to supply a current in the reverse directionof the current with which a reaction product is formed is applied one ormore times, whereby, ideally, the electrode surface can be restored tothe initial state at the time before the reaction product is depositedon the electrode surface as illustrated in FIG. 6F.

Unlike FIGS. 5A to 5F, FIGS. 7A to 7F are an example where a protectivefilm is formed on an electrode surface and illustrate a state where areaction product is deposited in a region not covered with theprotective film and is abnormally grown.

FIGS. 7A to 7C are schematic cross-sectional views sequentiallyillustrating the states of reaction products 232 a, 232 b, and 232 cabnormally grown and formed in a region of a surface of an electrode 230(typically, a negative electrode) that is not covered with a protectivefilm 234. A space between a pair of electrodes is filled with anelectrolytic solution 233. For the protective film 234, a single layerof a silicon oxide film, a niobium oxide film, or an aluminum oxide filmor a stack including any of the films is used.

FIG. 7A illustrates the state where a current is supplied between thenegative electrode and a positive electrode (not illustrated) during theperiod t1, and the reaction products 232 a are deposited on exposedportions of the electrode 230 serving as the negative electrode and aregrown abnormally.

FIG. 7B illustrates the state of the reaction product 232 b grown when acurrent is supplied between the negative electrode and the positiveelectrode during the period t2 (t2 is longer than t1). FIG. 7Cillustrates the state of the reaction product 232 c grown when a currentis supplied during the period t3 longer than the period t2.

After the state in FIG. 7C, a signal to supply a current in the reversedirection of the current with which a reaction product is formed isapplied to dissolve the reaction product. FIG. 7D illustrates the stateat the time immediately after a signal to supply a current in thereverse direction of the current with which a reaction product is formedis applied. As shown by arrows in FIG. 7D, a reaction product 232 d isdissolved from its tip or surface.

FIG. 7E illustrates the state where the reaction product is in themiddle of the dissolution due to further flow of the inversion pulsecurrent; the reaction product 232 d is dissolved from its tip or surfaceto be the reaction product 232 e smaller than the reaction product 232d.

In this manner, a signal to supply a current in the reverse direction ofthe current with which a reaction product is formed (i.e. a reversepulse current) is applied one or more times, whereby, ideally, theelectrode surface can be restored to the initial state at the timebefore the reaction product is deposited on the electrode surface asillustrated in FIG. 7F

As shown in FIGS. 7A to 7F, one embodiment of the invention disclosed inthis specification includes a first electrode, a protective filmcovering part of the first electrode, a second electrode, and anelectrolytic solution between the first electrode and the secondelectrode. When a current is supplied between the first electrode andthe second electrode, a reaction product is grown from a region of asurface of the first electrode that is not covered with the protectivefilm. This reaction product is dissolved by applying a signal to supplya current in the reverse direction of the current with which a reactionproduct is formed.

As described above, in the state illustrated in FIG. 5C, 6C, or 7C, adeposited reaction product, e.g., lithium or a whisker, can be dissolvedby applying an inversion pulse current as a signal with which a currentflows in the reverse direction of a charging current; thus, theelectrode surface can be restored to a normal state. Further, aninversion pulse current is supplied before the deposited lithium isseparated in charging, whereby the lithium is reduced in size ordissolved; thus, separation of the lithium can be prevented.

For example, a lithium metal deposited on a negative electrode of abattery cell causes a variety of defects in the battery cell, whichmight decrease the reliability of the battery cell. According to oneembodiment of the present invention, even when a lithium metal isdeposited on a negative electrode, it can be dissolved or made stable bysupplying an inversion pulse current while a battery cell is charged;thus, the reliability of the battery cell can be increased. This canincrease the capacity ratio of the battery cell, so that the size of thebattery cell can be reduced.

The power storage device 101 using such downsized battery cells can beinstalled indoors; specifically, the power storage device 101 can beplaced in the underfloor space 106, as illustrated in FIGS. 1A and 1B.Further, the power storage device 101 using such downsized battery cellscan have a longer lifetime; thus, replacement of the power storagedevice 101 is unnecessary. Further, the size of the power storage device101 may be increased to a size such that it can be placed in theunderfloor space 106.

The use of a plurality of battery cells of one embodiment of the presentinvention precludes an increase in the volume of the power storagedevice 101 as large as an increase in that of a conventional one; thus,the power storage device 101 does not need to be provided outdoors andcan be placed in the underfloor space 106. When being placed in theunderfloor space 106 as described above, the power storage device 101can be prevented from being exposed to rain or the like, inhibitingdeterioration of the power storage device 101 due to moisture. Further,even when the outside air is at a low temperature (e.g., in a minustemperature range), deterioration of the power storage device 101 can beinhibited because the power storage device 101 is provided indoors. Thiscan further increase the lifetime of the power storage device 101.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 3

In this embodiment, a power storage system and a power storage device ofone embodiment of the present invention will be described with referenceto FIG. 8, FIGS. 9A to 9C, and FIG. 10.

First, a power storage system of one embodiment of the present inventionwill be described with reference to FIG. 8, and then, a power storagedevice of one embodiment of the present invention will be described withreference to FIG. 10.

FIG. 8 illustrates an example of a power storage system 500 of oneembodiment of the present invention. As illustrated in FIG. 8, the powerstorage device 101 of one embodiment of the present invention isprovided in the underfloor space 106 of the building 100.

The power storage device 101 is provided with a control device 110, andthe control device 110 is electrically connected, via wirings, to adistribution board 503, a power storage controller (also referred to asa control device) 505, an indicator 506, and a router 509.

Power is supplied from a commercial power source 501 to the distributionboard 503 through a service wire mounting portion 510. Moreover, poweris supplied to the distribution board 503 from the power storage device101. Power supplied to the distribution board 503 is supplied to ageneral load 507 and a high power load 508 through an outlet (notillustrated).

The general load 507 is, for example, an electrical device such as a TVor a personal computer. The high power load 508 is, for example, anelectrical device such as a microwave, a refrigerator, or an airconditioner.

The power storage controller 505 includes a measuring portion 511, apredicting portion 512, and a planning portion 513. The measuringportion 511 has a function of measuring the amount of power consumed bythe general load 507 and the high power load 508 during a day (forexample, from midnight to midnight). The measuring portion 511 may alsohave a function of measuring the amount of power of the power storagedevice 101 and the amount of power supplied from the commercial powersource 501. The predicting portion 512 has a function of predicting thedemand for power consumed by the general load 507 and the high powerload 508 during a day on the basis of the amount of power consumed bythe general load 507 and the high power load 508 during the previousday. The planning portion 513 has a function of making a charge anddischarge plan of the power storage device 101 on the basis of thedemand for power predicted by the predicting portion 512.

The indicator 506 can show the amount of power consumed by the generalload 507 and the high power load 508 that is measured by the measuringportion 511. An electrical device such as a TV or a personal computercan also show it through the router 509. Further, a portable electronicterminal such as a smartphone or a tablet can also show it through therouter 509. Moreover, the indicator 506, the electrical device, and theportable electronic terminal can also show, for example, the demand forpower depending on a time period (or per hour) that is predicted by thepredicting portion 512.

FIG. 9A shows an example of the amount of power consumed by the generalload 507 and the high power load 508 that is measured by the measuringportion 511. In the bar graph shown in FIG. 9A, the horizontal axisrepresents time, and the vertical axis represents the amount of powerconsumed by the general load 507 and the high power load 508 that ismeasured by the measuring portion 511. The amount of power shown in FIG.9A is the sum of the amount of power supplied from the power storagedevice 101 and the amount of power supplied from the commercial powersource.

Some electric power companies adopt a system where the electric rate ishigh in a time period when the demand for power is high and the electricrate is low in a time period when the demand for power is low. FIG. 9Bshows an example of a rate structure of an electric power company. Someelectric power companies set different electric rates depending on timeperiods as shown in a circle graph of FIG. 9B. For example, the electricrate is low in the night time from 11 p.m. to 7 a.m., the electric rateis high in the day time from 10 a.m. to 5 p.m., and the electric ratefor the morning time and the evening time is an intermediate ratebetween the electric rate for the night time and the electric rate forthe day time.

Thus, the power storage system 500 is preferably set by the powerstorage controller 505 at least so that the power storage device 101 isnot charged and is only discharged in the day time when the electricrate is high and it is charged from the commercial power source 501 inthe night time when the electric rate is low, as shown in FIG. 9C. Thatis to say, in the day time, power is supplied to the general load 507and the high power load 508 from the power storage device 101, while inthe night time, power is supplied to the general load 507, the highpower load 508, and the power storage device 101 from the commercialpower source 501. The power storage device 101 is charged in the nighttime when power consumption of the general load 507 and the high powerload 508 is low, and is discharged in the day time when powerconsumption of the general load 507 and the high power load 508 is high,whereby the electricity charge can be low and an economic merit can beobtained.

Further, the power storage system 500 can be set by the power storagecontroller 505 so that depending on the demand for power in, forexample, the day time on the next day, charging the power storage device101 is started in the evening time and is terminated by the end of thenight time. Specifically, the power storage system 500 can be set sothat the power storage device 101 is charged for 10 hours from 9 p.m. to7 a.m. Alternatively, the power storage system 500 can be set so thatcharging the power storage device 101 is started in the night time andis terminated by the end of the morning time. Specifically, the powerstorage system 500 can be set so that the power storage device 101 ischarged for 10 hours from 11 p.m. to 9 a.m. The time for charging thepower storage device 101 can be set as appropriate in accordance withthe demand for power in the day time.

The power storage controller 505 can select, regardless of a timeperiod, supplying power from the power storage device 101 to the generalload 507 and the high power load 508 or supplying power from thecommercial power source 501 to the general load 507 and the high powerload 508. The power storage controller 505 has a function of supplyingpower to the general load 507 and the high power load 508 from the powerstorage device 101 regardless of a time period, when power supply fromthe commercial power source 501 is stopped or reduced, for example.

FIG. 10 illustrates an example of the power storage device 101 of oneembodiment of the present invention. As illustrated in FIG. 10, thepower storage device 101 includes at least the control device 110, aDC/DC converter 305, a first battery cell group 301, and a secondbattery cell group 302. The power storage device 101 further includes aprotective circuit 313, a protective circuit 314, a current detector315, a current detector 316, a switch 303, and a switch 304.

The control device 110 illustrated in FIG. 10 is electrically connectedto the power storage controller 505 illustrated in FIG. 8.

The DC/DC converter 305 is preferably a bi-directional DC/DC converter.One terminal of the DC/DC converter 305 is electrically connected to oneterminal of the switch 303 and one terminal of the first battery cellgroup 301. The other terminal of the DC/DC converter 305 is electricallyconnected to one terminal of the switch 304 and one terminal of thesecond battery cell group 302. The other terminal of the switch 303 andthe other terminal of the switch 304 are electrically connected to oneterminal of an AC/DC converter 306. The other terminal of the AC/DCconverter 306 is electrically connected to a terminal 321 (a powersource or a load).

The battery cell group 301 includes m battery cells (battery cells 311_1to 311_m) (m is a natural number) that are connected in series. Thebattery cell group 302 includes n battery cells (battery cells 312_1 to312_n) (n is a natural number) that are connected in series. The numberof battery cells in the battery cell group 301 is preferably equal to,but may be different from, that of battery cells in the battery cellgroup 302.

Serially connecting the plurality of battery cells in such a manner canincrease the output voltage of the power storage device 101. Forexample, the output voltage of one lithium-ion secondary battery isapproximately 3.2 V. When each of the battery cell group 301 and thebattery cell group 302 includes 100 battery cells that are connected inseries, for example, the output voltage of the power storage device 101can be increased to approximately 320 V.

The plurality of battery cells each includes a first electrode and asecond electrode, and includes at least an electrolytic solution betweenthe first electrode and the second electrode. One of the first electrodeand the second electrode is a positive electrode and the other is anegative electrode.

As illustrated in FIG. 10, the power storage device 101 includes acurrent detector and a protective circuit that determines and controlsthe states of all the battery cells. In FIG. 10, the protective circuit313 is electrically connected to the battery cells 311_1 to 311_m, andthe protective circuit 314 is electrically connected to the batterycells 312_1 to 312_n. Specifically, the protective circuit 313 iselectrically connected to the positive electrode and the negativeelectrode of each of the battery cells 311_1 to 311_m. The protectivecircuit 314 is electrically connected to the positive electrode and thenegative electrode of each of the battery cells 312_1 to 312_n.

One terminal of the current detector 315 is electrically connected tothe other terminal of the first battery cell group 301, and the otherterminal of the current detector 315 is grounded. One terminal of thecurrent detector 316 is electrically connected to the other terminal ofthe second battery cell group 302, and the other terminal of the currentdetector 316 is grounded.

The control device 110 controls each of the switch 303, the switch 304,the DC/DC converter 305, the AC/DC converter 306, the protective circuit313, the protective circuit 314, the current detector 315, and thecurrent detector 316. The control device 110 has a function ofterminating charging when the voltage of any of the battery cellsincreases to a predetermined voltage (e.g., 4.35 V for a lithium-ionsecondary battery) in charging, for example, by controlling theprotective circuit 313, the protective circuit 314, the current detector315, and the current detector 316. The control device 110 also has afunction of terminating discharging when the voltage of any of thebattery cells decreases to a predetermined voltage (e.g., 2.3 V for alithium-ion secondary battery) in discharging. The control device 110has a function of restricting or terminating charge and discharge whenthe temperature of any of the plurality of battery cells becomes out ofa predetermined temperature range. Thus, the control device 110 controlsthe protective circuit 313, the protective circuit 314, the currentdetector 315, and the current detector 316, whereby deterioration of thepower storage device 101 can be inhibited, resulting in a higher levelof safety.

Next, the case of charging the power storage device of one embodiment ofthe present invention, the case of supplying an inversion pulse currentfor a short time during charging of the power storage device, and thecase of discharging the power storage device will be described. In thisembodiment, an example will be described in which an AC voltage from theterminal 321 is converted into a DC voltage and the battery cell group301 and the battery cell group 302 are charged with the DC voltage, andthen, the DC voltage of the battery cell group 301 and the battery cellgroup 302 is converted into an AC voltage and the AC voltage is outputfrom the terminal 321.

The power storage device 101 is charged and discharged according to thecharge and discharge plan made by the planning portion 513 illustratedin FIG. 8.

<In the Case of Charging Power Storage Device 101>

First, the case of charging the power storage device 101 will bedescribed. In charging the power storage device 101, the switch 303 isturned on and the switch 304 is turned off, an AC current supplied fromthe terminal 321 is converted into a DC current by the AC/DC converter306, part of the DC current (e.g., half of the DC current) is suppliedto the battery cell group 302 through the switch 303 and the DC/DCconverter 305, and the rest of the DC current (e.g., the other half ofthe DC current) is supplied to the battery cell group 301 through theswitch 303. The ratio of a current supplied to the battery cell group301 to a current supplied to the battery cell group 302 can becontrolled as appropriate by the DC/DC converter 305.

Alternatively, the switch 303 and the switch 304 are turned on, an ACcurrent supplied from the terminal 321 is converted into a DC current bythe AC/DC converter 306, part of the DC current (e.g., half of the DCcurrent) is supplied to the battery cell group 301 through the switch303, and the rest of the DC current (e.g., the other half of the DCcurrent) is supplied to the battery cell group 302 through the switch304. The above manner may be employed to charge the battery cell group301 and the battery cell group 302.

<In the Case of Supplying Inversion Pulse Current while Charging PowerStorage Device 101>

Next, the case of supplying an inversion pulse current to the battercell group 301 or the battery cell group 302 while the power storagedevice 101 is charged will be described. The term “supplying aninversion pulse current” in this embodiment mean that a current is madeto flow in the reverse direction of a current with which a reactionproduct is deposited on the first electrode or the second electrode.

For example, in the case of supplying an inversion pulse current to thebattery cell group 302, the switch 303 is turned on and the switch 304is turned off. Thus, an AC current supplied from the terminal 321 isconverted into a DC current by the AC/DC converter 306, and the DCcurrent flows to the battery cell group 301. At this time, a currentfrom the battery cell group 302 also flows to the battery cell group 301through the DC/DC converter 305. As a result, the sum of a current fromthe AC/DC converter 306 and a current from the battery cell group 302 issupplied to the battery cell group 301. The time for one inversion pulsecurrent supply to the battery cell group 302 is longer than or equal to0.1 seconds and shorter than or equal to 3 minutes, and is typicallylonger than or equal to 3 seconds and shorter than or equal to 30seconds. After the inversion pulse current is supplied to the batterycell group 302, a DC current from the AC/DC converter 306 is supplied tothe battery cell group 302.

By thus repeating charging the battery cell group 302 and supplying aninversion pulse current, even if a reaction product is deposited on thefirst electrode or the second electrode of each battery cell, it can bedissolved. Thus, deterioration of the electrode of each battery cell canbe inhibited.

In the case of supplying an inversion pulse current to the battery cellgroup 301, the switch 303 is turned off and the switch 304 is turned on.Thus, an AC current supplied from the terminal 321 is converted into aDC current by the AC/DC converter 306, and the DC current flows to thebattery cell group 302. At this time, a current from the battery cellgroup 301 also flows to the battery cell group 302 through the DC/DCconverter 305. As a result, the sum of a current from the AC/DCconverter 306 and a current from the battery cell group 301 is suppliedto the battery cell group 302. The time for one inversion pulse currentsupply to the battery cell group 301 is longer than or equal to 0.1seconds and shorter than or equal to 3 minutes, and is typically longerthan or equal to 3 seconds and shorter than or equal to 30 seconds.After the inversion pulse current is supplied to the battery cell group301, a DC current from the AC/DC converter 306 is supplied to thebattery cell group 301.

By thus repeating charging the battery cell group 301 and supplying aninversion pulse current, even if a reaction product is deposited on thefirst electrode or the second electrode of each battery cell, it can bedissolved. Thus, deterioration of the electrode of each battery cell canbe inhibited.

The battery cell group 301 and the battery cell group 302 are chargedaccording to the charge and discharge plan made by the planning portion513 in the power storage controller 505 illustrated in FIG. 8.

Even if a reaction product (e.g., lithium metal) is deposited on thefirst electrode or the second electrode of each battery cell, it can bedissolved by repeatedly supplying an inversion pulse current in acharging period in the above manner. Thus, deterioration of theelectrode of each battery cell can be inhibited. Further, the capacityratio of each battery cell can be increased, so that the size of thebattery cell can be reduced.

Thus, the significantly downsized power storage device 101 that usessuch downsized battery cells can be installed indoors; specifically, thepower storage device 101 can be placed in the underfloor space 106.Further, the power storage device 101 using such downsized battery cellscan have a longer lifetime; thus, replacement of the power storagedevice 101 is unnecessary. Further, the size of the power storage device101 may be increased to a size such that it can be placed in theunderfloor space 106.

<In the Case of Discharging Power Storage Device 101>

Next, the case of discharging the power storage device 101 will bedescribed. In discharging the power storage device 101, the switch 303is turned on, the switch 304 is turned off, a DC current from thebattery cell group 301 is converted into an AC current by the AC/DCconverter 306, a DC current from the battery cell group 302 through theDC/DC converter 305 is converted into an AC current by the AC/DCconverter 306, and the AC current is supplied to the general load 507and the high power load 508 through the distribution board 503.

Alternatively, the switch 303 is turned off, the switch 304 is turnedon, a DC current from the battery cell group 302 is converted into an ACcurrent by the AC/DC converter 306, a DC current from the battery cellgroup 301 through the DC/DC converter 305 is converted into an ACcurrent by the AC/DC converter 306, and the AC current is supplied tothe general load 507 and the high power load 508 through thedistribution board 503.

Even if lithium metal is deposited on the negative electrode of any ofthe plurality of battery cells, the reaction product is dissolved andthe growth of the reaction product is inhibited by supplying aninversion pulse current in a charging period of the battery cells in theabove manner; thus, an increase in the resistance of the negativeelectrode can be inhibited. This can inhibit deterioration of thebattery cell, can prevent a reduction in the capacity of the batterycell due to charge or discharge, enables the battery cell or the like tobe controlled with low power consumption, can improve the reliability ofthe battery cell or the like, or leads to a higher level of safety ofthe battery cell.

Thus, inhibiting deterioration of each battery cell leads to inhibitionof deterioration of the power storage device 101, can prevent areduction in the capacity of the power storage device 101 due to chargeor discharge, enables the power storage device 101 to be controlled withlow power consumption, can improve the reliability of the power storagedevice 101 or the like, or leads to a higher level of safety of thepower storage device 101.

The use of the power storage device 101 in the power storage system 500illustrated in FIG. 8 enables planned charge and discharge of the powerstorage device 101. Thus, the power storage device 101 is charged in thenight time when power consumption of the general load 507 and the highpower load 508 is low, and is discharged in the day time when powerconsumption of the general load 507 and the high power load 508 is high,whereby the electricity charge can be low and an economic merit can beobtained.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 4

In this embodiment, the battery cell described in Embodiment 1 and amanufacturing method thereof will be described with reference to FIGS.11A and 11B, FIGS. 12A and 12B, and FIGS. 13A to 13C.

First, a positive electrode of the battery cell will be described withreference to FIGS. 11A and 11B.

A positive electrode 400 includes a positive electrode current collector401 and a positive electrode active material layer 402 formed over thepositive electrode current collector 401 by a coating method, a CVDmethod, a sputtering method, or the like, for example. Although FIG. 11Aillustrates an example of providing the positive electrode activematerial layers 402 so that the positive electrode current collector 401with a sheet shape (or a strip-like shape) is sandwiched therebetween,one embodiment of the present invention is not limited to this example.The positive electrode active material layer 402 may be provided overonly one of surfaces of the positive electrode current collector 401.Further, although the positive electrode active material layer 402 isprovided over the whole positive electrode current collector 401 in FIG.11A, the positive electrode active material layer 402 may be providedover part of the positive electrode current collector 401. For example,a structure may be employed in which the positive electrode activematerial layer 402 is not provided in a portion where the positiveelectrode current collector 401 is connected to a positive electrodetab.

The positive electrode current collector 401 can be formed using amaterial that has high conductivity and is not alloyed with a carrierion of lithium or the like, such as a metal typified by stainless steel,gold, platinum, zinc, iron, copper, aluminum, or titanium, or an alloythereof. Alternatively, an aluminum alloy to which an element whichimproves heat resistance, such as silicon, titanium, neodymium,scandium, or molybdenum, is added can be used. Still alternatively, ametal element which forms silicide by reacting with silicon can be used.Examples of the metal element which forms silicide by reacting withsilicon include zirconium, titanium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, cobalt, nickel, and the like.The positive electrode current collector 401 can have a foil-like shape,a plate-like shape (sheet-like shape), a net-like shape, apunching-metal shape, an expanded-metal shape, or the like asappropriate. The positive electrode current collector 401 preferably hasa thickness of greater than or equal to 10 μm and less than or equal to30 μm.

FIG. 11B is a schematic view illustrating a longitudinal cross sectionof the positive electrode active material layer 402. The positiveelectrode active material layer 402 includes positive electrode activematerial particles 403, graphenes 404 as a conductive additive, and abinder 405.

Examples of the conductive additive are acetylene black (AB), ketjenblack, graphite (black lead) particles, and carbon nanotubes in additionto graphene described later. Here, the positive electrode activematerial layer 402 using the graphenes 404 is described as an example.

The positive electrode active material particles 403 are made ofsecondary particles having an average particle diameter or a particlediameter distribution, which are obtained in such a way that materialcompounds are mixed at a predetermined ratio and baked and the resultingbaked product is crushed, granulated, and classified by an appropriatemeans. Therefore, the positive electrode active material particles 403are schematically illustrated as spheres in FIG. 11B; however, the shapeof the positive electrode active material particle 403 is not limited tothis shape.

For the positive electrode active material particles 403, a materialinto/from which lithium ions can be inserted and extracted can be used.For example, a material with an olivine crystal structure, a layeredrock-salt crystal structure, or a spinel crystal structure can be used.

As an olivine-type compound, a complex oxide represented by LiMPO₄(general formula) (M is one or more of Fe(II), Mn(II), Co(II), andNi(II)) can be given. Typical examples of LiMPO₄ (general formula) areLiFePO₄, LiNiPO₄, LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄,LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄, LiNi_(a)Co_(b)PO₄,LiNi_(a)Mn_(b)PO₄ (a+b≦1, 0<a<1, and 0<b <1), LiFe_(c)Ni_(d)Co_(e)PO₄,LiFe_(c)Ni_(d)Mn_(e)PO₄, LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≦1, 0<c<1, 0<d<1,and 0<e<1), and LiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≦1, 0<f<1, 0<g<1,0<h<1, and 0<i<1).

LiFePO₄ is particularly preferable because it properly has propertiesnecessary for the positive electrode active material, such as safety,stability, high capacity density, high potential, and the existence oflithium ions which can be extracted in initial oxidation (charge).

Examples of a material with a layered rock-salt crystal structure arelithium cobalt oxide (LiCoO₂), LiNiO₂, LiMnO₂, Li₂MnO₃, NiCo-containingcomposite oxide (general formula: LiNi_(x)Co_(1-x)O₂ (0<x<1)) such asLiNi_(0.8)Co_(0.2)O₂, NiMn-containing composite oxide (general formula:LiNi_(x)Mn_(1-x)O₂(0<x<1)) such as LiNi_(0.5)Mn_(0.5)O₂,NiMnCo-containing composite oxide (also referred to as NMC) (generalformula: LiNi_(x)Mn_(y)Co_(1-x-y)O₂ (x>0, y>0, x+y<1)) such asLiNi_(1/3)Mn_(1/3)Co_(1/3)O₂, Li(Ni_(0.8)Co_(0.15)Al_(0.05))O₂, andLi₂MnO₃—LiMO₂ (M=Co, Ni, or Mn).

Examples of a material with a spinel crystal structure are LiMn₂O₄,Li_(1+x)Mn_(2-x)O₄, Li(MnAl)₂O₄, and LiMn_(1.5)Ni_(0.5)O₄.

It is preferable to add a small amount of lithium nickel oxide (LiNiO₂or LiNi_(1-x)MO₂ (M=Co, Al, or the like)) to a material with a spinelcrystal structure which contains manganese, such as LiMn₂O₄, becauseadvantages such as minimization of the elution of manganese and thedecomposition of an electrolytic solution can be obtained.

Alternatively, a complex oxide represented by Li_((2-j))MSiO₄ (generalformula) (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II), 0≦j≦2)can be used for the positive electrode active material. Typical examplesof Li_((2-j))MSiO₄ (general formula) are Li_((2-j))FeSiO₄,Li_((2-j))NiSiO₄, Li_((2-j))CoSiO₄, Li_((2-j))MnSiO₄,Li_((2-j))Fe_(k)Ni_(l)SiO₄, Li_((2-j))Fe_(k)Co_(l)SiO₄,Li_((2-j))Fe_(k)Mn_(l)SiO₄, Li_((2-j))Ni_(k)Co_(l)SiO₄,Li_((2-j))Ni_(k)Mn_(l)SiO₄ (k+l≦1, 0<k<1, and 0<l<1),Li_((2-j))Fe_(m)Ni_(n)Co_(q)SiO₄, Li_((2-j))Fe_(m)Ni_(n)Mn_(q)SiO₄,Li_((2-j))Ni_(m)Co_(n)Mn_(q)SiO₄ (m+n+q≦1, 0<m<1, 0<n<1, and 0<q<1), andLi_((2-j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≦1, 0<r<1, 0<s<1,0<t<1,and 0<u<1).

Still alternatively, a nasicon compound expressed by A_(x)M₂(XO₄)₃(general formula) (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, Nb, or Al, X=S, P,Mo, W, As, or Si) can be used for the positive electrode activematerial. Examples of the nasicon compound are Fe₂(MnO₄)₃, Fe₂(SO₄)₃,and Li₃Fe₂(PO₄)₃. Further alternatively, a compound expressed byLi₂MPO₄F, Li₂MP₂O₇, or Li₅MO₄ (general formula) (M=Fe or Mn), aperovskite fluoride such as NaF₃ or FeF₃, a metal chalcogenide (asulfide, a selenide, or a telluride) such as TiS₂ or MoS₂, a materialwith an inverse spinel crystal structure such as LiMVO₄, a vanadiumoxide (V₂O₅, V₆O₁₃, LiV₃O₈, or the like), a manganese oxide, an organicsulfur, or the like can be used as the positive electrode activematerial.

Other examples of the positive electrode active material that can beused are an active material containing an alkali metal such as sodium orpotassium, an active material containing an alkaline-earth metal such ascalcium, strontium, barium, beryllium, or magnesium, and an activematerial containing a rare-earth element. Examples of the rare-earthelement are yttrium, scandium, lanthanum, cerium, praseodymium,neodymium, promethium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

In the case where carrier ions are alkali metal ions other than lithiumions, or alkaline-earth metal ions, the positive electrode activematerial particles 403 may be formed using, instead of the abovecompound or oxide containing lithium, a compound or oxide containing analkali metal (e.g., sodium or potassium), or an alkaline-earth metal(e.g., calcium, strontium, barium, beryllium, or magnesium).

In the case of using an active material containing sodium for thepositive electrode active material particles 403, for example, NaMn₂O₄,NaNiO₂, NaCoO₂, NaFeO₂, NaNi_(0.5)Mn_(0.5)O₂, NaCrO₂, or NaFeO₂ can beused. Alternatively, a fluorophosphate such as Na₂FePO₄F, Na₂VPO₄F,Na₂MnPO₄F, Na₂CoPO₄F, or Na₂NiPO₄F can be used. Still alternatively, aborate such as NaFeBO₄ or Na₃Fe₂(BO₄)₃ can be used.

Note that although not illustrated, a carbon layer may be provided on asurface of the positive electrode active material particle 403. A carbonlayer provided on a surface of the positive electrode active materialparticle 403 can increase the conductivity of the electrode. Thepositive electrode active material particle 403 can be coated with acarbon layer by mixing a carbohydrate such as glucose into the positiveelectrode active material particle in baking the positive electrodeactive material particle.

In addition, the graphene 404 that is added as a conductive additive tothe positive electrode active material layer 402 can be formed byreducing graphene oxide.

Note that graphene in this specification and the like refers tosingle-layer graphene or multilayer graphene including two or more andhundred or less layers. Single-layer graphene refers to a one-atom-thicksheet of carbon molecules having π bonds. Graphene oxide refers to acompound formed by oxidizing such graphene. When graphene oxide isreduced to form graphene, oxygen contained in the graphene oxide is notentirely released and part of the oxygen remains in the graphene. Whenthe graphene contains oxygen, the proportion of the oxygen, which ismeasured by XPS, is higher than or equal to 2 at. % and lower than orequal to 20 at. %, preferably higher than or equal to 3 at. % and lowerthan or equal to 15 at. %.

In the case where graphene is multilayer graphene including grapheneobtained by reducing graphene oxide, the interlayer distance betweengraphenes is greater than or equal to 0.34 nm and less than or equal to0.5 nm, preferably greater than or equal to 0.38 nm and less than orequal to 0.42 nm, more preferably greater than or equal to 0.39 nm andless than or equal to 0.41 nm. In general graphite, the interlayerdistance between single-layer graphenes is 0.34 nm. Since the interlayerdistance between the graphenes used for the power storage device of oneembodiment of the present invention is longer than that in generalgraphite, carrier ions can easily transfer between the graphenes inmultilayer graphene.

Graphene oxide can be formed by an oxidation method called a Hummersmethod, for example.

The Hummers method is as follows: a sulfuric acid solution of potassiumpermanganate, a hydrogen peroxide solution, or the like is mixed intographite powder to cause oxidation reaction; thus, a dispersioncontaining a graphite oxide is formed. Through the oxidation of carbonof graphite, functional groups such as an epoxy group, a carbonyl group,a carboxyl group, or a hydroxyl group are bonded in the graphite oxide.Accordingly, the interlayer distance between adjacent graphenes of aplurality of graphenes in graphite oxide is longer than the interlayerdistance of graphite, so that the graphite oxide can be easily separatedinto thin pieces by interlayer separation. Then, ultrasonic vibration isapplied to the mixed solution containing the graphite oxide, so that thegraphite oxide whose interlayer distance is long can be cleaved toseparate graphene oxide and to form a dispersion containing the grapheneoxide. The solvent is removed from the dispersion containing thegraphene oxide, so that powdery graphene oxide can be obtained.

Note that a method for forming graphene oxide is not limited to theHummers method using a sulfuric acid solution of potassium permanganate;for example, the Hummers method using nitric acid, potassium chlorate,sodium nitrate, potassium permanganate, or the like or a method forforming graphene oxide other than the Hummers method may be employed asappropriate.

The graphite oxide may be separated into thin pieces by application ofultrasonic vibration, by irradiation with microwaves, radio waves, orthermal plasma, or by application of physical stress.

The formed graphene oxide has an epoxy group, a carbonyl group, acarboxyl group, a hydroxyl group, or the like. In graphene oxide in apolar solvent typified by NMP (also referred to as N-methylpyrrolidone,1-methyl-2-pyrrolidone, N-methyl-2-pyrrolidone, etc.), oxygen in afunctional group is negatively charged; hence, while interacting withNMP, graphene oxides repel each other and do not easily aggregate.Accordingly, graphene oxides in a polar solvent can be easily disperseduniformly.

The length of one side (also referred to as a flake size) of thegraphene oxide is greater than or equal to 50 nm and less than or equalto 100 μm, preferably greater than or equal to 800 nm and less than orequal to 20 μm.

As in the cross-sectional view of the positive electrode active materiallayer 402 in FIG. 11B, a plurality of the positive electrode activematerial particles 403 are coated with a plurality of the graphenes 404.One sheet-like graphene 404 is connected to a plurality of the positiveelectrode active material particles 403. In particular, since thegraphene 404 is in the form of a sheet, surface contact can be made insuch a way that the surfaces of the positive electrode active materialparticles 403 are partly wrapped with the graphene 404. Unlike aconductive additive in the form of particles, such as acetylene black,which makes point contact with a positive electrode active material, thegraphenes 404 are capable of surface contact with low contactresistance; accordingly, the electron conductivity of the positiveelectrode active material particles 403 and the graphenes 404 can beimproved without an increase in the amount of a conductive additive.

Further, surface contact is made between the plurality of graphenes 404.This is because graphene oxides with exceptional dispersibility in apolar solvent are used for the formation of the graphenes 404. A solventis removed by volatilization from a dispersion medium including grapheneoxides uniformly dispersed and graphene oxides are reduced to givegraphenes; hence, the graphenes 404 remaining in the positive electrodeactive material layer 402 partly overlap with each other and aredispersed such that surface contact is made, thereby forming an electronconduction path.

Some of the graphenes 404 are provided between the positive electrodeactive material particles 403. Further, the graphene 404 is an extremelythin film (sheet) made of a single layer of carbon molecules or stackedlayers thereof and thus is in contact with part of the surfaces of thepositive electrode active material particles 403 in such a way as totrace these surfaces. A portion of the graphene 404 which is not incontact with the positive electrode active material particles 403 iswarped between the positive electrode active material particles 403 andcrimped or stretched.

Consequently, the plurality of graphenes 404 form an electron conductionnetwork in the positive electrode 400. This maintains a path forelectric conduction between the positive electrode active materialparticles 403. Thus, when graphenes whose raw material is graphene oxideand which are formed by reduction performed after a paste is formed areused as a conductive additive, the positive electrode active materiallayer 402 with high electron conductivity can be formed.

Further, the proportion of the positive electrode active materialparticles 403 in the positive electrode active material layer 402 can beincreased because it is not necessary to increase the additive amount ofa conductive additive to increase contact points between the positiveelectrode active material particles 403 and the graphenes 404. This canincrease the discharge capacity of the battery cell.

The average diameter of a primary particle of the positive electrodeactive material particles 403 is less than or equal to 500 nm,preferably greater than or equal to 50 nm and less than or equal to 500nm. To make surface contact with a plurality of the positive electrodeactive material particles 403, the graphenes 404 preferably have sidesthe length of each of which is greater than or equal to 50 nm and lessthan or equal to 100 μm, more preferably greater than or equal to 800 nmand less than or equal to 20 μm.

As the binder 405 in the positive electrode active material layer 402,polyvinylidene fluoride (PVDF) as a typical example, polyimide,polytetrafluoroethylene, polyvinyl chloride, ethylene-propylene-dienepolymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber,fluorine rubber, polyvinyl acetate, polymethyl methacrylate,polyethylene, nitrocellulose, or the like can be used.

The above positive electrode active material layer 402 preferablyincludes the positive electrode active material particles 403 at greaterthan or equal to 90 wt % and less than or equal to 94 wt %, thegraphenes 404 as a conductive additive at greater than or equal to 1 wt% and less than or equal to 5 wt %, and the binder at greater than orequal to 1 wt % and less than or equal to 5 wt % with respect to thetotal weight of the positive electrode active material layer 402.

Next, a negative electrode of a battery cell will be described withreference to FIGS. 12A and 12B. A negative electrode 410 includes anegative electrode current collector 411 and a negative electrode activematerial layer 412 formed over the negative electrode current collector411 by a coating method, a CVD method, a sputtering method, or the like,for example. Although FIG. 12A illustrates an example of providing thenegative electrode active material layers 412 so that the negativeelectrode current collector 411 with a sheet shape (or a strip-likeshape) is sandwiched therebetween, one embodiment of the presentinvention is not limited to this example. The negative electrode activematerial layer 412 may be provided over only one of surfaces of thenegative electrode current collector 411. Further, although the negativeelectrode active material layer 412 is provided over the whole negativeelectrode current collector 411 in FIG. 12A, the negative electrodeactive material layer 412 may be provided over part of the negativeelectrode current collector 411. For example, a structure may beemployed in which the negative electrode active material layer 412 isnot provided in a portion where the negative electrode current collector411 is connected to a negative electrode tab.

The negative electrode current collector 411 can be formed using ahighly conductive material which is not alloyed with a carrier ion oflithium or the like, such as a metal typified by stainless steel, gold,platinum, zinc, iron, copper, or titanium or an alloy thereof.Alternatively, a metal element which forms silicide by reacting withsilicon can be used. Examples of the metal element which forms silicideby reacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, andthe like. The negative electrode current collector 411 can have afoil-like shape, a plate-like shape (sheet-like shape), a net-likeshape, a punching-metal shape, an expanded-metal shape, or the like asappropriate. The negative electrode current collector 411 preferably hasa thickness of 10 μm. to 30 μm inclusive.

FIG. 12B schematically illustrates part of a cross section of thenegative electrode active material layer 412. Although the negativeelectrode active material layer 412 includes a negative electrode activematerial 413 and a binder 415 in this embodiment, one embodiment of thepresent invention is not limited to this; the negative electrode activematerial layer 412 includes at least the negative electrode activematerial 413.

A material with which lithium can be dissolved and precipitated or amaterial into and from which lithium ions can be inserted and extractedcan be used for the negative electrode active material 413; for example,a lithium metal, a carbon-based material, an alloy-based material, orthe like can be used.

The lithium metal is preferable because of its low redox potential(3.045 V lower than that of a standard hydrogen electrode) and highspecific capacity per unit weight and per unit volume (3860 mAh/g and2062 mAh/cm³).

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, carbon black, and the like.

Examples of the graphite include artificial graphite such as meso-carbonmicrobeads (MCMB), coke-based artificial graphite, or pitch-basedartificial graphite and natural graphite such as spherical naturalgraphite.

Graphite has a low potential substantially equal to that of a lithiummetal (0.1 V to 0.3 V vs. Li/Li⁺) when lithium ions are intercalatedinto the graphite (while a lithium-graphite intercalation compound isformed). For this reason, a lithium-ion secondary battery can have ahigh operating voltage. In addition, graphite is preferable because ofits advantages such as relatively high capacity per unit volume, smallvolume expansion, low cost, and safety greater than that of a lithiummetal.

For the negative electrode active material 413, an alloy-based materialwhich enables charge-discharge reactions by an alloying reaction and adealloying reaction with lithium can be used. In the case where carrierions are lithium ions, a material containing at least one of Al, Si, Ge,Sn, Pb, Sb, Bi, Ag, Zn, Cd, In, Ga, and the like can be used forexample. Such elements have higher capacity than carbon. In particular,silicon has a significantly high theoretical capacity of 4200 mAh/g. Forthis reason, silicon is preferably used as the negative electrode activematerial 413. Examples of the alloy-based material using such elementsinclude SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂, Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂,Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃,InSb, SbSn, and the like.

Alternatively, for the negative electrode active material 413, an oxidesuch as titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂),lithium-graphite intercalation compound (Li_(x)C₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) can be used.

Still alternatively, for the negative electrode active material 413,Li_(3-x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive material 413 and thus the negative electrode active material 413can be used in combination with a material for a positive electrodeactive material which does not contain lithium ions, such as V₂O₅ orCr₃O₈. Note that in the case of using a material containing lithium ionsas a positive electrode active material, the nitride containing lithiumand a transition metal can be used for the negative electrode activematerial by extracting the lithium ions contained in the positiveelectrode active material in advance.

Alternatively, a material which causes a conversion reaction can be usedas the negative electrode active material 413; for example, a transitionmetal oxide which does not cause an alloy reaction with lithium, such ascobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), may beused. Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, or CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃. Note that any of the fluorides can be used as the positiveelectrode active material particles 403 because of its high potential.

Although the negative electrode active material 413 is illustrated as aparticulate substance in FIG. 12B, the shape of the negative electrodeactive material 413 is not limited thereto. The negative electrodeactive material 413 can have a given shape such as a plate shape, a rodshape, a cylindrical shape, a powder shape, or a flake shape. Further,the negative electrode active material 413 may have a three-dimensionalshape such as unevenness on a surface of a plate shape, fine unevennesson a surface, or a porous shape.

The negative electrode active material layer 412 may be formed by acoating method in the following manner: a conductive additive (notillustrated) or a binding agent is added to the negative electrodeactive material 413 to form a negative electrode paste; and the negativeelectrode paste is applied to the negative electrode current collector411 and dried.

The negative electrode active material layer 412 may be predoped withlithium in such a manner that, for example, a lithium layer is formed ona surface of the negative electrode active material layer 412 by asputtering method. Alternatively, lithium foil is provided on thesurface of the negative electrode active material layer 412, whereby thenegative electrode active material layer 412 can be predoped withlithium.

Further, graphene (not illustrated) is preferably formed on the surfaceof the negative electrode active material 413. For example, in the caseof using silicon as the negative electrode active material 413,reception and release of carrier ions in charge and discharge cyclesgreatly change the volume of silicon. This decreases adhesion betweenthe negative electrode current collector 411 and the negative electrodeactive material layer 412, resulting in degradation of batterycharacteristics caused on charge and discharge. In view of this,graphene is preferably formed on the surface of the negative electrodeactive material 413 containing silicon because this makes it possible toinhibit a decrease in adhesion between the negative electrode currentcollector 411 and the negative electrode active material layer 412 dueto a change in the volume of silicon in charge and discharge cycles,which helps reduce degradation of battery characteristics.

Graphene formed on the surface of the negative electrode active material413 can be formed by reducing graphene oxide in a manner similar to thatof the method for forming the positive electrode. As the graphene oxide,the above-described graphene oxide can be used.

Further, a coating film 414 of an oxide or the like may be formed on thesurface of the negative electrode active material 413. A surface filmformed by decomposition of an electrolytic solution, or the like incharging cannot release electric charge used in the formation, andtherefore forms irreversible capacity. In contrast, the film of an oxideor the like provided on the surface of the negative electrode activematerial 413 in advance can reduce or prevent generation of irreversiblecapacity.

As the coating film 414 covering the negative electrode active material413, an oxide film of any one of niobium, titanium, vanadium, tantalum,tungsten, zirconium, molybdenum, hafnium, chromium, aluminum, andsilicon and an oxide film containing lithium and any one of theseelements can be used. The coating film 414 formed using such a film issufficiently dense as compared with a conventional surface film formedon the surface of a negative electrode by a decomposition product of anelectrolytic solution.

The product of the electric resistivity and the thickness of the coatingfilm 414 at 25° C. is greater than or equal to 20 Ωm·m, preferablygreater than or equal to 200 Ωm·m. When the product of the electricresistivity and the thickness of the coating film 414 at 25° C. isgreater than or equal to 20 Ωm·m, the decomposition reaction between thenegative electrode active material and an electrolytic solution can bereduced. Further, when the product of the electric resistivity and thethickness of the coating film 414 at 25° C. is greater than or equal to200 Ωm·m, the decomposition reaction between the negative electrodeactive material and an electrolytic solution can be inhibited.

A sol-gel method can be employed to form the coating film 414 coveringthe negative electrode active material 413, for example. The sol-gelmethod is a method for forming a thin film in such a manner that asolution of metal alkoxide, a metal salt, or the like (also referred toas a treatment liquid) is changed into a gel that has lost its fluidityby a hydrolysis reaction and a polycondensation reaction and the gel isbaked. Since a thin film is formed from a liquid phase in the sol-gelmethod, raw materials can be mixed uniformly on the molecular scale. Forthis reason, by adding a negative electrode active material such asgraphite to a raw material of a metal oxide film, the active materialcan be easily dispersed into the gel. In such a manner, the coating film414 can be formed on the surface of the negative electrode activematerial 413.

Alternatively, the coating film covering the negative electrode activematerial 413 may be formed in such a manner that a conductive additiveand/or a binder is added to the negative electrode active material 413to form a negative electrode paste, the negative electrode paste isapplied to the negative electrode current collector 411 and dried toform a coated electrode, the coated electrode is immersed in a treatmentliquid, and then a hydrolysis reaction and a polycondensation reactionoccur. This method allows a plurality of negative electrode activematerial particles to be in contact with each other and permits thecoating film to cover part or the whole of the surface except a regionwhere the plurality of negative electrode active material particles arein contact with each other. This can inhibit the reduction decompositionof an electrolytic solution, inhibiting formation of a decompositionproduct of the electrolytic solution on the negative electrode activematerial particles due to the reduction decomposition of theelectrolytic solution.

The use of the coating film 414 can prevent a decrease in the capacityof a power storage device.

Next, the structure of a battery cell that can be used for a powerstorage device will be described with reference to FIGS. 13A to 13C.

FIG. 13A is an external view of a coin-type (single-layer flat type)lithium-ion battery cell, part of which illustrates a cross-sectionalview of part of the coin-type lithium-ion battery cell.

In a coin-type battery cell 550, a positive electrode can 551 doublingas a positive electrode terminal and a negative electrode can 552doubling as a negative electrode terminal are insulated from each otherand sealed by a gasket 553 made of polypropylene or the like. A positiveelectrode 554 includes a positive electrode current collector 555 and apositive electrode active material layer 556 provided in contact withthe positive electrode current collector 555. A negative electrode 557includes a negative electrode current collector 558 and a negativeelectrode active material layer 559 provided in contact with thenegative electrode current collector 558.

A separator 560 and an electrolytic solution (not illustrated) areprovided between the positive electrode active material layer 556 andthe negative electrode active material layer 559.

The negative electrode 557 includes the negative electrode currentcollector 558 and the negative electrode active material layer 559. Thepositive electrode 554 includes the positive electrode current collector555 and the positive electrode active material layer 556.

For the positive electrode 554, the negative electrode 557, theseparator 560, and the electrolytic solution, the above-describedmembers can be used.

For the positive electrode can 551 and the negative electrode can 552, ametal having corrosion resistance to an electrolytic solution, such asnickel, aluminum, or titanium, an alloy of such a metal, or an alloy ofsuch a metal and another metal (e.g., stainless steel or the like) canbe used. Alternatively, the positive electrode can 551 and the negativeelectrode can 552 are preferably covered with nickel, aluminum, or thelike in order to prevent corrosion caused by the electrolytic solution.The positive electrode can 551 and the negative electrode can 552 areelectrically connected to the positive electrode 554 and the negativeelectrode 557, respectively.

The negative electrode 557, the positive electrode 554, and theseparator 560 are immersed in the electrolytic solution. Then, asillustrated in FIG. 13A, the positive electrode can 551, the positiveelectrode 554, the separator 560, the negative electrode 557, and thenegative electrode can 552 are stacked in this order with the positiveelectrode can 551 positioned at the bottom, and the positive electrodecan 551 and the negative electrode can 552 are subjected to pressurebonding with the gasket 553 interposed therebetween. In such a manner,the coin-type battery cell 550 is fabricated.

It is preferable that, for example, a graphite electrode and lithiumiron phosphate (LiFePO₄) be used for the negative electrode 557 and thepositive electrode 554 of the battery cell 550, respectively. In thatcase, the safety of the battery cell 550 and the safety of the powerstorage device 101 in FIGS. 1A and 1B that includes the battery cell 550can be increased.

Next, an example of a laminated battery cell will be described withreference to FIG. 13B. In FIG. 13B, a structure inside the laminatedbattery cell is partly exposed for convenience.

A laminated battery 570 using a laminate film as an exterior body andillustrated in FIG. 13B includes a positive electrode 573 including apositive electrode current collector 571 and a positive electrode activematerial layer 572, a negative electrode 576 including a negativeelectrode current collector 574 and a negative electrode active materiallayer 575, a separator 577, an electrolytic solution (not illustrated),and an exterior body 578. The separator 577 is provided between thepositive electrode 573 and the negative electrode 576 in the exteriorbody 578. The exterior body 578 is filled with the electrolyticsolution. Although the one positive electrode 573, the one negativeelectrode 576, and the one separator 577 are used in FIG. 13B, thebattery cell may have a layered structure in which positive electrodesand negative electrodes are alternately stacked and separated byseparators.

For the positive electrode 573, the negative electrode 576, theseparator 577, and the electrolytic solution (an electrolyte and asolvent), the above-described members can be used.

In the laminated battery 570 illustrated in FIG. 13B, the positiveelectrode current collector 571 and the negative electrode currentcollector 574 also serve as terminals (tabs) for an electrical contactwith an external portion. For this reason, each of the positiveelectrode current collector 571 and the negative electrode currentcollector 574 is arranged so that part of the positive electrode currentcollector 571 and part of the negative electrode current collector 574are exposed on the outside the exterior body 578.

As the exterior body 578 in the laminated battery 570, for example, alaminate film having a three-layer structure in which a highly flexiblemetal thin film of aluminum, stainless steel, copper, nickel, or thelike is provided over a film formed of a material such as polyethylene,polypropylene, polycarbonate, ionomer, or polyamide, and an insulatingsynthetic resin film of a polyamide-based resin, a polyester-basedresin, or the like is provided as the outer surface of the exterior bodyover the metal thin film can be used. With such a three-layer structure,permeation of the electrolytic solution and a gas can be blocked and aninsulating property can be obtained.

Next, an example of a rectangular battery cell will be described withreference to FIG. 13C. A wound body 580 illustrated in FIG. 13C includesa negative electrode 581, a positive electrode 582, and a separator 583.The wound body 580 is obtained by winding a sheet of a stack in whichthe negative electrode 581 overlaps with the positive electrode 582 withthe separator 583 provided therebetween. The wound body 580 is coveredwith a rectangular sealing can or the like; thus, a rectangular batterycell is fabricated. Note that the number of stacks each including thenegative electrode 581, the positive electrode 582, and the separator583 may be determined as appropriate depending on required capacity ofthe battery cell and the volume of an element.

As in a cylindrical battery cell, in the rectangular battery cell, thenegative electrode 581 is connected to a negative electrode tab (notillustrated) through one of a terminal 584 and a terminal 585, and thepositive electrode 582 is connected to a positive electrode tab (notillustrated) through the other of the terminal 584 and the terminal 585.

Although the coin-type battery cell, the laminated battery cell, and therectangular battery cell are described above as examples of the batterycell, battery cells with a variety of shapes can be used. Further, astructure in which a plurality of positive electrodes, a plurality ofnegative electrodes, and a plurality of separators are stacked or woundmay be employed.

As the separators included in the battery cells illustrated in FIGS. 13Ato 13C, a porous insulator such as cellulose (paper), polypropylene(PP), polyethylene (PE), polybutene, nylon, polyester, polysulfone,polyacrylonitrile, polyvinylidene fluoride, or tetrafluoroethylene canbe used. Alternatively, nonwoven fabric of a glass fiber or the like, ora diaphragm in which a glass fiber and a polymer fiber are mixed may beused.

The electrolytic solution used for each of the battery cells illustratedin FIGS. 13A to 13C is preferably a nonaqueous solution (solvent)containing an electrolyte (solute).

As a solvent for the electrolytic solution, an aprotic organic solventis preferably used. For example, one of ethylene carbonate (EC),propylene carbonate (PC), butylene carbonate, chloroethylene carbonate,vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate(DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methylformate, methyl acetate, methyl butyrate, 1,3-dioxane, 1,4-dioxane,dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyldiglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, andsultone can be used, or two or more of these solvents can be used in anappropriate combination in an appropriate ratio.

When a gelled high-molecular material is used as the solvent for theelectrolytic solution, safety against liquid leakage and the like isimproved. Further, a battery cell can be thinner and more lightweight.Typical examples of the gelled high-molecular material include asilicone gel, an acrylic gel, an acrylonitrile gel, polyethylene oxide,polypropylene oxide, a fluorine-based polymer, and the like.

Alternatively, the use of one or more of ionic liquids (room temperatureionic liquids) that has non-flammability and non-volatility as thesolvent for the electrolytic solution can prevent a battery cell fromexploding or catching fire even when the battery cell internally shortsout or the internal temperature increases due to overcharging or thelike. Thus, the safety of the battery cell can be increased. With theuse of the ionic liquid as the solvent for the electrolytic solution,the battery cell can favorably operate even in a low temperature range(minus temperature range) as compared with the case where an organicsolvent is used as the solvent for the electrolytic solution.

As an electrolyte dissolved in the above solvent, one of lithium saltssuch as LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄,Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃,LiC(C₂F₅S0₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂)(CF₃SO₂), and LiN(C₂F₅SO₂)₂ canbe used, or two or more of these lithium salts can be used in anappropriate combination in an appropriate ratio.

Although the case where carrier ions are lithium ions in the aboveelectrolyte is described, carrier ions other than lithium ions can beused. Note that when carrier ions other than lithium ions are alkalimetal ions or alkaline-earth metal ions, instead of lithium in the abovelithium salts, an alkali metal (e.g., sodium or potassium), analkaline-earth metal (e.g., calcium, strontium, barium, beryllium, ormagnesium) may be used for an electrolyte.

Further, as shown in FIGS. 2A and 3A, ideally, the reactions of lithiuminsertion and extraction at the negative electrode is equivalent to thereactions of lithium insertion and extraction at the positive electrode.Accordingly, in the case where the capacity per volume of the negativeelectrode is 1 and that of the positive electrode is 1, the idealcapacity ratio is 100%. However, in practice, the capacity per volume ofthe negative electrode is generally higher than that of the positiveelectrode. In FIGS. 13A to 13C, the size of a piece of the graphite isgreater than or equal to 9 μm and less than or equal to 30 μm, and alayer of the graphite has a thickness of greater than or equal to 50 μmand less than or equal to 100 μm, The size of a particle of the lithiumiron phosphate is greater than or equal to 50 nm and less than or equalto 200 nm, and a layer of the lithium iron phosphate has a thickness ofgreater than or equal to 60 μm and less than or equal to 110 μm. Inaddition, as the capacity ratio is closer to 100%, the capacity easilydecreases and an abnormal behavior is easily shown.

Supplying an inversion pulse current as a signal with which a currentflows in the reverse direction of a charging current can prevent adecrease in capacity and an abnormal behavior even when the capacityratio is 60% or as high as 85%. This indicates that an abnormal behaviorcaused by lithium deposition is inhibited. Further, the capacity ratiocan be close to 100%, resulting in a great improvement in capacity percell volume. That is, application of a signal with which a current flowsin the reverse direction of a charging current, during charge enablesreduction in the size of a battery, in addition to prevention ofprogress in the battery deterioration or an increase in reliability dueto recovery from a deteriorated state. In addition, rapid charge anddischarge of the battery can be performed.

When lithium is deposited and thus the length of a whisker is increased,a positive electrode and a negative electrode might be short-circuited;however, the supply of an inversion pulse current during charge caninhibit the lithium deposition and can desirably dissolve a deposit,resulting in an increase in the reliability of a battery. Further,employing a structure of supplying an inversion pulse current duringcharging allows a structure without a separator provided to prevent apositive electrode and a negative electrode from being short-circuited,which can reduce the cost of materials and shorten the manufacturingprocess owing to the simplified structure.

The use of a plurality of such battery cells enables fabrication of thepower storage device 101 illustrated in FIGS. 1A and 1B. Specifically,battery cells which operate according to the mechanisms described inEmbodiment 1 are connected in series. Further, parallely connectingunits each including the plurality of battery cells connected in seriescan increase the capacity of the power storage device 101. Even whenhaving high capacity and large volume, the power storage device 101 canbe placed in an underfloor space surrounded by a base and a floor of abuilding as illustrated in FIGS. 1A and 1B. Since the power storagedevice 101 can be placed in the underfloor space 106, the power storagedevice 101 does not need to be provided outdoors. When being placed inthe underfloor space 106 as described above, the power storage device101 can be prevented from being exposed to rain or the like, inhibitingdeterioration of the power storage device 101 due to moisture. Further,even when the outside air is at a low temperature (e.g., in a minustemperature range), deterioration of the power storage device 101 can beinhibited because the power storage device 101 is provided indoors. Thiscan further increase the lifetime of the power storage device 101.

This embodiment can be freely combined with any of the otherembodiments.

EXAMPLE 1

In this example, measurement results of the cycle characteristics of abattery cell of one embodiment of the present invention will bedescribed. In this example, battery cells having four differentproportions (capacity ratios; 85%, 80%, 60%, and 40%) of volume capacityof a positive electrode to volume capacity of a negative electrode asdesign conditions of the battery cells were evaluated.

First, structures and fabrication methods of coin-type battery cellsused in this example will be described. A battery cell fabricated tohave a capacity ratio of 85% is referred to as battery cell A; a batterycell fabricated to have a capacity ratio of 80% is referred to asbattery cell B; a battery cell fabricated to have a capacity ratio of60% is referred to as battery cell C; and a battery cell fabricated tohave a capacity ratio of 40% is referred to as battery cell D. Six foreach battery cell were fabricated.

Positive electrodes used for battery cells A to D were framed in thefollowing manner. First. NMP was prepared as a dispersion medium,graphene oxide (GO) was dispersed in the NMP at 0.6 wt % as a conductiveadditive, lithium iron phosphate (which was coated with carbon; alsoreferred to as C/LiFePO₄) was added at 91.4 wt % as a positive electrodeactive material, and then, the mixture was kneaded until it had theconsistency of thick paste. After PVDF was added at 8 wt % as a binderto the mixture of the graphene oxide and the lithium iron phosphate, NMPwas added as a dispersion medium and mixed, whereby a positive electrodepaste was formed.

The positive electrode paste was applied to a positive electrode currentcollector (20-μm-thick aluminum), dried at 80° C. in an air atmospherefor 40 minutes, and then dried at 170° C. in a reduced atmosphere for 10hours, whereby the positive electrode in which a positive electrodeactive material layer was formed over the positive electrode currentcollector was formed.

Here, the positive electrode used for battery cell A included thepositive electrode active material layer with a thickness of 58 μm; thepositive electrode used for battery cell B included the positiveelectrode active material layer with a thickness of 72 μm, the positiveelectrode used for battery cell C included the positive electrode activematerial layer with a thickness of 55 μm; and the positive electrodeused for battery cell D included the positive electrode active materiallayer with a thickness of 61 μm.

An electrode sold by TAKUMI GIKEN Co., Ltd. was used as each of negativeelectrodes of battery cells A to C. Copper foil was used as a negativeelectrode current collector, mesocarbon microbeads (MCMBs) with a graindiameter of 9 μm were used as a negative electrode active material,conductive graphite was used as a conductive additive, and PVDF was usedas a binder. The weight ratio of the negative electrode active materialto the conductive additive and the binder in a negative electrode activematerial layer was 79:14:7.

Here, the negative electrode used for battery cell A included thenegative electrode active material layer with a thickness of 65 μm; thenegative electrode used for battery cell B included the negativeelectrode active material layer with a thickness of 86 μm; and thenegative electrode used for battery cell C included the negativeelectrode active material layer with a thickness of 86 μm.

A negative electrode of battery cell D was formed in the followingmanner. First, silicon ethoxide, ethyl acetoacetate, and toluene weremixed and stirred to form a Si(OEt)₄ toluene solution. At this time, theamount of the silicon ethoxide was determined so that the proportion ofsilicon oxide formed later to graphite (mesocarbon microbeads (MCMBs)each with a diameter of 9 μm) serving as the negative electrode activematerial was 1 wt %. The compounding ratio of this solution was asfollows: the Si(OEt)₄ was 3.14×10⁻⁴ mol; the ethyl acetoacetate,6.28×10⁻⁴ mol; and the toluene, 2 ml.

Next, the Si(OEt)₄ toluene solution to which graphite was added wasstirred in a dry room. Then, the solution was held at 70° C. in a humidenvironment for 3 hours so that the Si(OEt)₄ in the Si(OEt)₄ toluenesolution to which the graphite was added was hydrolyzed and condensed.In other words, the Si(OEt)₄ in the solution gradually reacted withmoisture in the air, so that hydrolysis reaction gradually occurred, andthe Si(OEt)₄ after the hydrolysis was condensed by dehydration reactionfollowing the hydrolysis reaction. In such a manner, gelled silicon wasattached to the surfaces of graphite particles to form a net-likestructure of a C—O—Si bond.

Then, baking was performed at 500° C. in a nitrogen atmosphere for threehours, whereby graphite covered with silicon oxide was formed.

The graphite covered with 1 wt % of silicon oxide and PVDF as a binderwere mixed to form a negative electrode paste, and the negativeelectrode paste was applied to a negative electrode current collectorand dried, so that a negative electrode active material layer wasformed. In this case, the weight ratio of the graphite to the PVDF was90:10. As a solvent, NMP was used.

Here, the thickness of the negative electrode active material layer ofthe negative electrode used for battery cell D was 106 μm.

In each of battery cells A to D, an electrolytic solution in which ECand DEC were used as a nonaqueous solvent at a weight ratio of 3:7 and 1M of LiPF₆ was dissolved as an electrolyte was used.

As a separator, a 25-μm-thick porous polypropylene film was used. Theseparator was impregnated with the above electrolytic solution.

A positive electrode can and a negative electrode can were formed ofstainless steel (SUS). As a gasket, a spacer or a washer was used.

Next, the positive electrode can, the positive electrode, the separator,the negative electrode, the gasket, and the negative electrode can werestacked, and the positive electrode can and the negative electrode canwere crimped to each other with a “coin cell crimper”. Six for each ofcoin-type battery cells A to D were fabricated.

Table 1 shows design conditions of battery cells A to D. Note that acapacity ratio in Table 1 is a value obtained by dividingsingle-electrode theoretical capacity of the positive electrode bysingle-electrode theoretical capacity of the negative electrode. InTable 1, GO represents graphene oxide and AB represents acetylene black.In addition, the term “content” refers to the amount of the activematerial per unit area of the active material layer.

TABLE 1 Single- electrode Electrode theoretical Active ConductiveThickness density Content capacity Capacity Electrode material additiveBinder [μm] [g/cm³] [mg/cm²] [mAh/cm²] ratio Battery Positive C/LiFePO₄GO PVDF 58 1.71 9.4 1.6 0.84 cell A electrode Content rate 91.4 0.6 8Negative Graphite Conductive PVDF 65 0.99 5.1 1.9 electrode graphiteContent rate 79   14   7 Battery Positive C/LiFePO₄ GO PVDF 72 1.92 12.62.1 0.78 cell B electrode Content rate 91.4 0.6 8 Negative GraphiteConductive PVDF 86 1.08 7.4 2.8 electrode graphite Content rate 79  14   7 Battery Positive C/LiFePO₄ GO PVDF 55 1.89 9.5 1.6 0.59 cell Celectrode Content rate 91.4 0.6 8 Negative Graphite Conductive PVDF 861.08 7.4 2.8 electrode graphite Content rate 79   14   7 BatteryPositive C/LiFePO₄ GO PVDF 61 1.68 9.4 1.6 0.37 cell D electrode Contentrate 91.4 0.6 8 Negative Graphite AB PVDF 106 1.22 11.6 4.3 electrode(MCMB 9 μm) with coating Content rate 90   0   10 

In each of battery cells A to D, the thicknesses of the negativeelectrode current collector, the positive electrode current collector,and the separator were 18 μm, 20 μm, and 25 μm, respectively.

FIG. 14A shows the thicknesses of components, i.e., the negativeelectrode current collector, the negative electrode active materiallayer, the separator, the positive electrode active material layer, andthe positive electrode current collector, of each of battery cells A toD. FIG. 14B shows the cell capacity of each of battery cells A to Dobtained by calculation. Note that in FIG. 14B, the irreversiblecapacity was calculated by dividing 10% of the single-electrodetheoretical capacity of the negative electrode by the total thickness ofthe components. Further, the cell capacity was calculated in such amanner that the single-electrode theoretical capacity of the positiveelectrode was divided by the total thickness of the components and theirreversible capacity was subtracted from the obtained value.

Next, the cycle characteristics of battery cells A to D were measured.In each of battery cells A to D, an inversion pulse current was suppliedto three of the six battery cells and was not supplied to the othersduring charging.

In the case of supplying an inversion pulse current to the battery cellin charging, the charging was performed at a charge rate of 1 C (170mA/g) and was terminated when the constant current (CC) was 4.0 V. Notethat 1 C means the amount of current per unit weight for fully charginga battery cell (each of the evaluation cells, here) in an hour. In thisspecification, when LiFePO₄ is used for the positive electrode of thebattery cell and the theoretical capacity of the LiFePO₄ is 170 mAh/g, acharging current of 170 mA is 1 C (170 mA/g) assuming that the weight ofthe LiFePO₄ as the positive electrode is 1 g. In this case, an idealbattery is fully charged in an hour. Provided that 1 g of LiFePO₄ is apositive electrode, charging at a charging rate of 2 C means thatcharging is performed by supplying a charging current of 340 mA for 0.5hours. Further, a signal with which a current flows in the reversedirection of a charging current was applied per certain amount ofcharged power (10 mAh/g). Ten seconds of inversion pulse current supplywas repeated during charging with the rate set to 1 C (170 mA/g).Discharging was performed at a discharge rate of 1 C and was terminatedwhen the constant current (CC) was 2.0 V. The charging and thedischarging were regarded as one cycle, and the cycle characteristicswere measured.

In the case of not supplying an inversion pulse current to the batterycell during charging, the charging was performed at a charge rate of 1 C(170 mA/g) and was terminated when the constant current (CC) was 4.0 V.Discharging was performed at a discharge rate of 1 C and was terminatedwhen the constant current (CC) was 2.0 V. The charging and thedischarging were regarded as one cycle, and the cycle characteristicswere measured.

FIGS. 15A and 15B show results of the cycle characteristics of batterycells A and B, respectively, and FIGS. 16A and 16B show results of thecycle characteristics of the battery cells C and D, respectively. Ineach of FIGS. 15A and 15B and FIGS. 16A and 16B, the horizontal axisrepresents the number of cycles [times] and the vertical axis representsdischarge capacity [mAh/g]. Moreover, in FIGS. 15A and 15B and FIGS. 16Aand 16B, bold lines indicate results when an inversion pulse current issupplied to the battery cells during charging and thin lines indicateresults when an inversion pulse is not supplied to the battery cellsduring charging.

As shown in FIGS. 15A and 15B and FIGS. 16A and 16B, in the case of notsupplying an inversion pulse current to the battery cells during thecharging, battery cell A with a capacity ratio of 85% and battery cell Cwith a capacity ratio of 60% exhibited abnormal behavior. In contrast,in the case of supplying an inversion pulse current to the battery cellsduring the charging, stable cycle characteristics are exhibited underall the conditions.

This application is based on Japanese Patent Application serial no.2013-040595 filed with Japan Patent Office on Mar. 1, 2013, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A power storage system comprising: a control device;and a power storage device configured to supply power to a load, whereinthe control device includes: a measuring portion configured to measurean amount of power consumed by the load, a predicting portion configuredto predict the demand for power consumed by the load on the basis of theamount of power consumed by the load, and a planning portion configuredto make a charge and discharge plan of the power storage device on thebasis of the demand for power predicted by the predicting portion. 3.The power storage system according to claim 2, wherein the load is atelevision, a personal computer, a microwave, a refrigerator, or an airconditioner.
 4. The power storage system according to claim 2, whereinthe predicting portion is configured to predict the demand for powerconsumed by the load during a day on the basis of the amount of powerconsumed by the load during the previous day.
 5. The power storagesystem according to claim 2, further comprising an indicator configuredto show the amount of power consumed by the load.
 6. The power storagesystem according to claim 2, further comprising a router through whichthe amount of power consumed by the load is configured to be shown on asmartphone or a tablet.
 7. The power storage system according to claim2, further comprising a power storage controller configured to controlthe power storage device to be discharged in day time and be charged innight time.
 8. A power storage system comprising: an AC/DC converter; acontrol device; and a power storage device configured to supply power toa load, wherein the control device includes: a measuring portionconfigured to measure an amount of power consumed by the load, apredicting portion configured to predict the demand for power consumedby the load on the basis of the amount of power consumed by the load,and a planning portion configured to make a charge and discharge plan ofthe power storage device on the basis of the demand for power predictedby the predicting portion, wherein the power storage device comprises: aDC/DC converter, a first battery cell group, and a second battery cellgroup, wherein one terminal of the DC/DC converter is electricallyconnected to one terminal of the AC/DC converter and one terminal of thefirst battery cell group, and wherein the other terminal of the DC/DCconverter is electrically connected to the one terminal of the AC/DCconverter and one terminal of the second battery cell group.
 9. Thepower storage system according to claim 8, wherein the first batterycell group and the second battery cell group each include a plurality ofbattery cells connected in series in which each battery cell includes afirst electrode, a second electrode, and an electrolytic solutionbetween the first electrode and the second electrode.
 10. The powerstorage system according to claim 8, further comprising: a first switchbetween the one terminal of the AC/DC converter and the one terminal ofthe first battery cell group; and a second switch between the oneterminal of the AC/DC converter and the one terminal of the secondbattery cell group.
 11. The power storage system according to claim 8,further comprising: a first current detector electrically connected tothe other terminal of the first battery cell group; and a second currentdetector electrically connected to the other terminal of the secondbattery cell group.
 12. The power storage system according to claim 8,wherein the load is a television, a personal computer, a microwave, arefrigerator, or an air conditioner.
 13. The power storage systemaccording to claim 8, wherein the predicting portion is configured topredict the demand for power consumed by the load during a day on thebasis of the amount of power consumed by the load during the previousday.
 14. The power storage system according to claim 8, furthercomprising an indicator configured to show the amount of power consumedby the load.
 15. The power storage system according to claim 8, furthercomprising a router through which the amount of power consumed by theload is configured to be shown on a smartphone or a tablet.
 16. Thepower storage system according to claim 8, further comprising a powerstorage controller configured to control the power storage device to bedischarged in day time and be charged in night time.